Turns out it was Springfield, Oregon, all along

Matt Groening finally "solves" one of the great pop culture mysteries of our time in an interview with Smithsonian Magazine:

OK, why do the Simpsons live in a town called Springfield? Isn’t that a little generic?Springfield was named after Springfield, Oregon. The only reason is that when I was a kid, the TV show “Father Knows Best” took place in the town of Springfield, and I was thrilled because I imagined that it was the town next to Portland, my hometown. When I grew up, I realized it was just a fictitious name. I also figured out that Springfield was one of the most common names for a city in the U.S. In anticipation of the success of the show, I thought, “This will be cool; everyone will think it’s their Springfield.” And they do.

You’ve never said it was named after Springfield, Oregon, before, have you?
I don’t want to ruin it for people, you know? Whenever people say it’s Springfield, Ohio, or Springfield, Massachusetts, or Springfield, wherever, I always go, “Yup, that’s right.”

This isn't the first time Matt indicated which Springfield was the real one, FWIW. I believe he indicated as such either at the end or shortly after the contest was done in conjunction with "The Simpsons" movie in 2007.